Page last updated at 23:14 GMT, Friday, 7 May 2010 00:14 UK

How big is the Deepwater Horizon oil spill?

Elena Egawhary
BBC News

Thousands of tonnes of oil have poured into the Gulf of Mexico after the disaster at the Deepwater Horizon oil rig over two weeks ago. But how does this leak compare with the largest offshore spills on record?

Size of world spills

The current size of the Deepwater Horizon spill is hard to measure exactly, but attempts can be made to estimate it.

Based on oil flow calculations from the National Oceanic and Atmospheric Administration, Dr Simon Boxall, a marine pollution expert, says a total current spill can be estimated at about 7,000 to 10,000 tonnes of oil. (See factbox below for how this was worked out.)

But such estimates should always carry a caveat, he says, as these can be affected by factors such as the condition of the rig, the well and the quality of information available.

After the incident on 21 April, first estimate of the volume of oil leaking to surface was about 1,000 barrels a day
On 29 April this was increased to 5,000 barrels a day and remains unchanged

In terms of lives lost (11 workers died in the rig explosion), financial cost and environmental damage, the Deepwater Horizon incident is clearly serious. But it is not one of the world's largest spills in terms of size alone.

In fact, based on the estimate above, it would not register in the largest 50 single incident, offshore oil spills that have occurred worldwide. Even the 1989 Exxon Valdez spill - despite the controversy and coverage - is not in the top 10.

However the potential for damage caused by Deepwater Horizon is apparent when looking at the events of June 1979 in the Bay of Campeche, also in the Gulf of Mexico.

In that spill, the exploratory oil well Ixtoc 1 suffered a blowout and wasn't capped until more than nine months later, having released 461,000 tonnes of oil in total.

With the current situation in the Gulf of Mexico still uncertain, Dr Boxall, of the University of Southampton, points out reasons for optimism.

A plan to place a giant funnel over the leak could change things dramatically, he says.

"They reckon they will reduce the flow by 80% to 90%. And while there is no such thing as a good oil spill, the environment can cope much better with 70 tonnes a day than with 700 tonnes a day."

Only two of the spills in our list of the world's largest originated from oil rig explosions, the rest are tanker-related.

The largest of these came also in 1979, from the Atlantic Empress. It collided with the Aegean Captain in the Caribbean sea, spilling 287,000 tonnes of oil.

But far bigger than any of these peacetime accidents is the amount of oil spilled in the immediate aftermath of the first Iraq War, 1991. Although not a single offshore spill, it saw massive oil leaks that easily dwarf Ixtoc 1 with an estimated 1.4 million to 1.5 million tonnes of oil released into the Persian Gulf by Iraqi forces as they retreated from Kuwait.

Map of oil spills
1. Ixtoc 1: 476,000 tonnes Bay of Campeche, Gulf of Mexico
2. Atlantic Empress: 287,000 tonnes Off Tobago, West Indies
3. Nowruz Oil Field: 272, 000 tonnes Persian Gulf
4. ABT Summer: 260,000 tonnes 700 nautical miles off Angola
5. Castillo de Bellver: 252,000 tonnes Off Saldanha Bay, South Africa
6. Amoco Cadiz: 223,000 tonnes off Brittany, France
7. Haven: 144,000 tonnes Genoa, Italy
8. Odyssey: 132,000 tonnes 700 nautical miles off Nova Scotia, Canada
9. Torrey Canyon: 119,000 tonnes Scilly Isles, UK
10. Sea Star: 115,000 tonnes Gulf of Oman
Exxon Valdez: 37,000 tonnes Prince William Sound, Alaska, US
Deepwater Horizon: 7,000 to 10,000 tonnes Gulf of Mexico

The biggest leaks are not necessarily the most environmentally destructive.

The tanker Exxon Valdez, which ran aground on Bligh Reef, Alaska ,in 1989, caused serious damage to the environment, killing thousands of seabirds as well as seals, sea otters, whales and fish. The remote location in sheltered waters only accentuated the problems.

The overall impact of an oil spill cannot be measured solely on size; weather conditions, the type of oil and the time it takes to stem the flow are just some of the many factors that also need to be considered.

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